Humics as an electron donor for anaerobic respiration

The possibility that microorganisms might use reduced humic substances (hum ics) as an electron donor for the reduction of electron accepters with a mo re positive redox potential was investigated. All of the Fe(III)- and humic s-reducing microorganisms evaluated were capable of oxidizing reduced humic s and/or the reduced humics analogue anthrahydroquinone-2,6,-disulphonate ( AHQDS), with nitrate and/or fumarate as the electron acceptor. These includ ed Geobacter metallireducens, Geobacter sulphurreducens, Geothrix fermentan s, Shewanella alga, Wolinella succinogenes and 'S. barnesii', Several of th e humics-oxidizing microorganisms grew in medium with AHQDS as the sole ele ctron donor and fumarate as the electron acceptor. Even though it does not reduce Fe(III) or humics, Paracoccus denitrificans could use AHQDS and redu ced humics as electron donors for denitrification, However, another denitri fier, Pseudomonas denitrificans, could not. AHQDS could also serve as an el ectron donor for selenate and arsenate reduction by W. succinogenes, Electr on spin resonance studies demonstrated that humics oxidation was associated with the oxidation of hydroquinone moieties in the humics, Studies with G. metallireducens and W. succinogenes demonstrated that the anthraquinone-2, 6-disulphonate (AQDS)/AHQDS redox couple mediated an interspecies electron transfer between the two organisms. These results suggest that, as microbia lly reduced humics enter less reduced zones of soils and sediments, the red uced humics may serve as electron donors for microbial reduction of several environmentally significant electron accepters.